New answers tagged interferometry
You can get information about size, shape and rotation of the emitting object. I would start here: http://cds.cern.ch/record/378753/files/9902020.pdf
When you move the mirror, you change the way the light has to go along, thus changing the time it needs to get to the interference surface. This means you change the phase shift of both light beams, too. It has nothing to do with different frequencies (you use only one light source, as I assume?).
If we assume the wave-fronts that initially enter the system to produce this image are symmetric, than the result is intuitively symmetric as all the components used to produce it are too. And anyone who looks at the images you have shown, will clearly see a symmetry about the Y-axis. Assuming they do not look for minuscule errors due to wave-front ...
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